I don't know, but one possibility is that they think it's easier to ensure the extremely high degree of safety with a separate parachute capsule. If the booste

I don't know, but one possibility is that they think it's easier to ensure the extremely high degree of safety with a separate parachute capsule. If the boosters can be recovered 99% of the time, but are destroyed 1% of the time, that would be plenty reliable to drastically cut the cost of the trip but obviously not reliable enough for humans. And it might not make sense to bring that reliability to something like 0.1% or 0.01%, which you need for humans, if it doubles the cost of the booster.